Method of stripping nickel from articles and the composition used therein



United States Patent Olfice 3,399,143 Patented Aug. 27, 1968 3,399,143 METHOD OF STRIPPING NICKEL FROM ARTICLES AND THE COMPOSITION USED THEREIN Leo J. Slominski, Bristol, Conn., assignor to MacDermid Incorporated, Waterbury, Conn., a corporation of Connecticut No Drawing. Continuation-impart of application Ser. No. 403,934, Oct. 14, 1964. This application Aug. 2, 1967, Ser. No. 657,781

11 Claims. (Cl. 252-792) ABSTRACT OF THE DISCLOSURE Compositions for stripping nickel from plating racks and the like, and process of using such compositions, which compositions consist essentially of nitric or combinations of nitric and sulfuric acid catalyzed by the inclusion of chloride ions, wherein the criticality of the chloride ion concentration in such systems is substantially ameliorated by the further inclusion of iodate ions, preferably in the ratio of about 1:10 by weight relative to the chloride. The iodate inclusion has the added eifect of suppressing nitrogenous fumes in the stripping reaction and has a synergistic effect in combination with the chloride on the stripping rate of the acid.

CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of my prior copending application Ser. No. 403,934, filed Oct. 14, 1964, entitled, Dissolution of Nickel Coatings, now abandoned.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to the dissolution of electrodeposited metallic coatings, and more particularly the dissolution of unwanted metallic deposits which have built up on the contact tips of the plating racks of electroplating apparatus.

These unwanted coatings introduce a number of problems in the use of the plating apparatus. Commonly the deposits are nodular in structure and accumulate to a degree which impairs the usefulness of the tips of the racks, frequently causing an increase in the number of plating rejects. In addition, as the nodules grow in size, their area increases so that a disproportionate amount of plating current is drawn by them, resulting in a waste of plated metal.

(2) Description of the prior art Heretofore, electroplaters have had recourse to two methods for removing the unwanted deposits in order to remedy the problem. The two methods consist of chemical dissolution and mechanical removal.

Because the unwanted deposit or coating is brittle, hammering of the contact tips of the racks usually shatters the deposit and removes it rather effectively. But repeated treatment of this type tends to damage the tips, especially when the racks are made of Phosphor bronze or some similar metal, as has been common practice. Then, too, many plating racks contain several hundred individual tips, which makes individual mechanical removal in this manner prohibitive in terms of labor cost.

To obviate this difliculty, there has been a trend in recent years towards greater use of stainless steel rack tips in the plating industry. Use of such equipment makes it possible to dissolve copper, nickel and other deposited metal coatings produced in the electrodeposition process by subjecting the plating tips to a solution of strong nitric acid. Because stainless is relatively unattacked by such acid, this offers a more acceptable and practical way for eliminating the unwanted accumulations.

However, use of nitric acid by itself provides a very slow rate of dissolution of nickel, and it has therefore been the practice to add a catalyst, more particularly a chloride ion as disclosed in US. Patent 2,200,486, when nickel is the predominant material of the deposit to be removed. The use of the chloride catalyst is specific to the dissolution of nickel, but its presence does not interfere with the use of the same solution for removal of other deposits, such as copper, cadmium, etc. But the catalyzed system presents the further problem that the chloride, in the presence of strong nitric acid, forms nitrosyl chloride through oxidation in solution. Nitrosyl chloride is a gas at temperatures above -5.S C. and its benefit to the stripping solution is therefore soon lost by volatilization, Such loss by evaporation is of course hastened by the normal agitation of the solution caused -by travel of the plating racks through the solution, if a plating system incorporates a stripping operation in its cycle, or by lowering and raising the racks into and from a stripping tank separate from the regular plating cycle. Also the evaporation is hastened by operation at temperatures which are well above the condensation temperature of the gas.

This loss of chloride is so serious that it falls below its minimum effective concentration by the time dissolution of only 12 to 19 ounces of nickel per gallon of solution has been achieved. Potentially, the nitric acid solution will accept up to 25 oz./gal. at 25 C. before saturation is reached. Thus the utilization of the stripping solution is substantially below that which it is possible and de-- sirable to achieve.

Moreover, maintenance of the chloride concentration by chemical analysis is so tedious as to be uneconomical as well as impractical in ordinary commercial plant operation, yet the chloride content is very critical. Attempt to obviate the task of close control of chloride, by addition initially of an excess in anticipation of the loss, results in virtually complete passivation of the nickel. For catalyzing purposes in the conventional concentrated nitric acid stripping solutions, the minimum effective concentration of chloride is about 0.01 mole/liter, while the maximum is about 0.07 mole/liter. This is a very narrow concentration range for a critical constituent, and the difficulties this narrow range imposes are multiplied by reason of the high volatility of the chloride constituent.

SUMMARY OF THE (INVENTION It is accordingly a principal object of this invention to overcome the foregoing defects and disadvantages of the methods heretofore used in removing unwanted electrodeposited nickel coatings on plating apparatus and the like. This is achieved by increasing the chloride tolerance of the stripping solution as hereinafter explained so that much greater concentrations can be used initially without incurring passivation. It is also a purpose to provide a system which is operative over much wider range of chloride concentrations for the stripping of nickel and one which at the same time remains useful for stripping deposits other than nickel.

It is a further object of the invention to suppress or reduce the evolution of dangerous gases, and more particularly nitrogen dioxide, resulting inherently in the use of the conventional nitric acid Stripping solutions.

It has now been found that iodic acid, and more particularly the iodate ion, will co-catalyze the conventional chloride-nitric acid system whereby chloride concentrations as high at 1.0 molar can readily be tolerated in nickel stripping operations. This represents something like a fifteen-fold increase in the upper critical limit over what could be used before. Additionally, the surprising discovery has been made that the chloride-iodate combination acts synergistically in that the nickel dissolution scription which follow as to the manner in which the invention can be utilized. For clarity, all concentrations are herein expressed in moles/liter. Concentrated nitric having a specific gravity of 1.42, is considered as 15.8 M (molar).

DESCRIPTION OF THE PREFERRED EMBODIMENTS The examples enumerated in the following tables give dissolution rates calculated from the weight loss of a nickel panel, one inch square, immersed in the respective stripping solutions maintained at 25 C, without agita tion. The dissolution rates given are in mils of nickel dissolved per minute (1 mil=0.001 inch).

For practical considerations, dissolution rates of less than about 0.1 mil/minute are generally unacceptable, since the rate decreases logarithmically, as a firstorder reaction, to impracticably low values with progressive exhaustion of the solution. The low limit for commercially significant dissolution rates, therefore, is considered to be one-tenth mil per minute for purposes of this disclosure.

In its preferred form, the invention contemplates the use of iodic acid as the co-catalyst in a chloride-catalyzed nitric acid system; but as already mentioned, it is the iodate ion which is effective without regard for its cation. Thus, in addition to iodic acid, any of the soluble iodate salts when in solution will ionize to form the iodate ion and are etfective. Furthermore hydriodic acid, iodides, and even elemental iodine, are oxidized to iodic acid by the highly oxidizing nitric acid-chloride catalyst system. Therefore any of these materials can be used as the source of iodate ion and function with good effectiveness.

Shown in the accompanying Table 1 is the variation in rate of dissolution for nickel obtained at difierent acid concentrations with constant, optimum chloride concentration, both with and without the co-catalyzing iodate.

TABLE 1 Without any With HNOa concencatalyst With .065 MCl- .065 M Cl-+ tration (M) (mil/min.) (mil/min.) .01 M I03 (mil/min.)

. 0038 0. 15 0. l7 0011 0. 43 0. 83 0014 0. 82 1. 8 0013 0. 28 0. 49 0022 026 029 0009 X Passive 0022 0004 1 Passive 0022 1 No measurable Weight loss in five minutes.

The system employed in making the foregoing'test consisted esesntially of nitric acid, either alone or in combination with the sources of chloride and iodate ions, in the appropriate cases. -It is apparent from the foregoing table that an optimum concentration for nitric acid in such system occurs at around 11.2 M, but that a range from somewhat below 9.5 M up to 15.8 M is usable. In addition to the fact that maximum dissolution rates are obtained in nitric acid solutions of 11.2 M, there rate is greatly increased over that obtained--through theare two other reasons for preferring this concentration. First of"all,this -is the approximate concentration of standard, 36 B. nitric acid. Secondly, such an acid solution at 25 C. provides a maximum for nickel solubility, namely about 25 oz./gal. At the higher nitric acid molarities, the nickel solubility decreases and in 15.8 M nitric acid the solubility of nickel is only about 15 oz./gal at 25 C.

As seen also from Table 1, the addition of the iodate ion increases the dissolution rate at all concentrations of nitric acid. And except at the highest acid concentration, the dissolution rate in the co-catalyzed system is more than twice that of the solution containing only the chloride catalyst. Not only is this advantage of greater dissolution rate obtained but, as will be presently shown, the troublesome criticality of the chloride ion concentration is dramatically alleviated, and etfective operation of the system becomes possible over a much wider range ofchloride ion concentrations than has been possible heretofore.

Some idea of the criticality of the chloride concentration in the conventional system can be obtained from Table 2. This table shows the relation between nickel dissolution rates and chloride concentrations at 25 C. in a nitric acid solution of 11.2 M, the system in this instance of course being without the benefit of the iodate of the invention.

As Table 2 clearly shows, there is a very sharp decrease in dissolution rate when the chloride concentration is above 0.065 M Which represents a preferred starting concentration in order to allow for depletion of the chloride during the course of the stripping operation. Yet care must be taken to avoid concentrations substantially above that level, as evidenced by the very poor rate of dissolution (high passivity) at 0.1 M in this conventional system.

Turning again to the present invention, it has been found that chloride molarities as high as 1.0 are possible, when the system also contains the iodate, while still maintaining dissolution rates for nickel in excess of the previously mentioned lower limit of 0.1 mil/minute. Conversely the chloride level can be allowed to become quite low (0.01 M) through evaporation losses, yet the dissolution rate of the system can still be maintained at an acceptable level by addition of a proper amount of iodate. In fact the dissolution rate is substantially proportional to the iodate concentration. This is illustrated in Table 3 which gives dissolution rates for different iodate concentrations in 11.2 M nitric acid at 25 C. containing 0.01 mole of chloride.

TABLE '3 Dissolution Iodate concentration (M): rate, mil/min. 0.00 0.21

1.0 (approximately) 3 For practical purposes, it is frequently desirable to prepare a dry admixture of a chloride and iodate which can be added to the nitric acid when it is desired to prepare the stripping solution. Shipping and storage problems are thus minimized. This can be easily accomplished by preparing a dry mixture of salts of these two catalysts; for example 2.7 parts by weight of sodium chloride to 1.0 part of potassium iodate. Such a mixture when added to concentrated nitric acid (9.5 M to 15.8 M) in amounts of 1 to oz./ gal. will provide an effective nickel stripping solution.

lit will be apparent to those skilled in the art that a nitric acid solution of 11.2 M will dissolve all commonly deposited metals except chromium and most of the precious metals. Even decorative chromium plate will generally be stripped in such a solution because it is thin and porous and therefore the usual underlying deposit of nickel is subject to attack. With the dissolution of the nickel substrate, the chromium plate is effectively removed by flaking off.

The presence of the iodate catalyst does not interfere with the normal action of nitric acid on metals other than nickel, so that the iodate-containing stripping solution is compatible for use in stripping such other metals. Moreover the presence of the iodate is advantageous in such cases even though it does not increase the rate of dissolution by reason of its effect in suppressing the evolution of the volatile nitrogenous decomposition products.

As previously indicated, the cation of the iodate is not important to the invention. Iodates which are useful and generally available commercially include, beside iodic acid, the sodium, potassium, barium and calcium salts of that acid. Likewise hydriodic acid and the corresponding alkali and alkaline earth metal salts, as well as elemental iodine, are oxidized in strong nitric acid to provide iodate ions, and these sources for the iodate are therefore useful. Similarly, the chloride may be provided by chlorine, hydrochloric acid or one of the alkali or alkaline earth metal salts thereof, including ammonium and magnesium.

While there appears to be no criticality from an operating standpoint for the upper concentration limit of the iodate, economic considerations dictate that 0.1 M represents a practical maximum consistent with good stripping effectiveness of the solution.

As already mentioned, in the systems described above nitric acid alone is employed as the acid source. It is possible, however, to obtain comparable results with a lower cost, more economical system in which a portion of the nitric acid is replaced by an equivalent (i.e., acid equivalent) amount of sulfuric acid. This will be apparent from the theoretical background considerations involved. In the system first described (i.e., nitric acid alone as the acid source), the overall reaction can be represented as follows:

It is obvious in this reaction that only of the nitric acid is involved in the oxidation; the remainder is necessary simply to supply the hydrogen ion needed for the reaction. This hydrogen ion requirement can be supplied by a leSS expensive acid than nitric, and sulfuric acid is the most suitable one from a cost as well as from an operability standpoint. 'Ihe stripping action in the substituted acid system where the maximum possible substitution of sulfuric for nitric acid is employed may be represented as follows:

In practice, the proportions present in Equation 2 re sult in a rate of reaction that is too slow to be of practical interest, since the reaction rate is proportional to the concentration of the nitric acid. Thus an equinormal ratio of nitric and sulfuric acid is about the minimum practical system. Such a system may be represented as follows:

A somewhat higher ratio is preferred in practice than that represented by the limiting condition in Equation 3 as the best compromise between speed and economy. Thus it has been found that relative acid concentrations of about 6.75 M HNO to 3.5 M H is optimum. The stripping rate in such a system, where the chloride ion concentration is 0.18 M, is 0.4 mil/minute at 25 C. without agitation. The chloride-iodate dependency remains the same as before.

What is claimed is:

1. A composition for stripping accumulated nickel deposits from plating apparatus and the like, which consists essentially of nitric acid, a source of chloride ion and a source of iodate ion, said chloride ion being present in concentrations of from 0.01 M to 1.0 M and said iodate ion being present in small but effective amount sufficient to accelerate the stripping rate and suppress the evolution of nitrogenous gases.

2. A composition as defined in claim 1, wherein the iodate ion concentration is a minimum of 0.001 M.

3. A composition as defined in claim 1, wherein the ratio of iodate ion concentration to chloride ion concentration is about 1:10.

4. A composition for stripping accumulated nickel deposits from plating apparatus and the like, which consists essentially of a 9.5 M to 15.8 M nitric acid containing chloride ions in concentrations of from 0.01 M to 1.0 M and iodate ions in concentrations of from 0.001 M to 0.1 M.

5. A composition as defined in claim 4, wherein a portion of the nitric acid is replaced by the acid equivalent of sulfuric acid in amount not to exceed retention of a .minimum limiting nitric acid concentration of 4.0 M in said composition.

6. An aqueous solution for stripping accumulated nickel deposits from plating apparatus, which consists essentially of 9.5 M to 15.8 M nitric acid, a soluble chlo ride selected from the group consisting of chlorine, hydrochloric acid and the sodium, potassium, ammonium, calcium, barium and magnesium salts thereof, in amount sufficient to provide a chloride ion concentration of 0.01 M to 1.0 M, and a soluble iodine compound selected from the group consisting of iodine, iodic and hydriodic acids and the sodium, potassium, barium and calcium salts of such acids, in amount sufiicient to provide an iodate ion concentration of from 0.001 M to 0.1 M in said solution.

7. An aqueous solution as defined in claim 6, wherein a portion of the nitric acid is replaced by the acid equivalent of sulfuric acid in amount not to exceed retention of a minimum limiting amount nitric acid concentration of 4.0 M in said solution.

8. An aqueous solution as defined in claim 7, wherein the concentrations of the components is approximately 6.75 M nitric acid, 3.5 M sulfuric acid, 0.18 hydrochloric acid and 0.018 iodic acid.

9. In the method of stripping nickel deposits from articles by immersing them in a concentrated nitric acid solution containing a chloride catalyst, the improvement which consists in adding to such stripping solution a small but effective amount of an iodine compound capable of providing iodate ions in said solution.

'10, The method of suppressing evolution of nit-rogenous fumes from a metal treating solution consisting essentially of nitric acid and a chloride,rwhich comprises adding to such solution an iodine compound capable of providing from about 0.001 to 011 m01e. per liter of iodate ion in such solution.

11. The method as defined in claim 10,'Where1n the treating solution contains up to equi-normal amount 0 sulfuric acid in substitution of nitric acid. a Y

8 References Cited -"UNYFED'"STATESPATENTS 11 2 QO4 6 5 9. pr q I I"T'TT" V"-.',Y'T'TY W 1057;764 10/1962 La Boda et al. 25 2 79.1 3,100,170 8/1963 Levy 156-18 3,322,673 5/1967 Slominski 134 41 RICHARD O. DEAN, Prjmary Examiner. 

